The present invention generally relates to fuel-fired forced air heating furnace apparatus and more particularly relates to recuperative heat exchanger apparatus used in condensing furnaces.
With the growing need to improve the overall energy efficiency of fuel-fired forced air heating furnaces, considerable design effort has been directed toward increasing the combustion gas-to-supply air heat transfer capability of their heat exchanger components. Traditionally, fuel-fired forced air heating furnaces have been provided with heat exchangers designed to extract only sensible heat from the combustion gases passing therethrough, and transfer a substantial portion of the extracted sensible heat to the air being forced externally across the heat exchanger.
Because only sensible heat is withdrawn from the combustion gases, no appreciable amount of condensation of the combustion gases occurs within the heat exchanger during furnace operation. This mode of heat transfer is commonly referred to as a "dry" or "nonrecuperative" process. The combustion gases exiting the heat exchanger, and discharged to atmosphere through a vent stack, are typically quite hot due to the appreciable amount of latent heat remaining therein. Accordingly, a considerable amount of available combustion gas heat is simply dumped to ambient, and the overall heat transfer efficiency of nonrecuperative heat exchangers is generally limited to about 85%.
To capture otherwise wasted latent combustion gas heat, recuperative or "condensing" type heat exchanger structures have been used in which a secondary or "wet" heat exchanger is connected in series with the primary or dry heat exchanger at its combustion gas discharge end. During furnace operation, the primary heat exchanger performs its usual task of extracting sensible heat from the combustion gas internally traversing it, and the secondary heat exchanger operates to extract primarily latent heat from the combustion gas, thereby considerably lowering the temperature of the combustion gas ultimately discharged into the vent stack. The use of condensing type primary/secondary heat exchangers of this type potentially raises the overall heat exchanger thermal efficiency to about 95%
Due to the substantially lowered temperature of combustion gas internally traversing the secondary heat exchange portion of the overall recuperative heat exchanger structure during furnace firing, condensate forms within the secondary heat exchange portion and must be continuously drained away. Typically, the condensate formed in the secondary heat exchange portion is drained away by gravity by mounting the recuperative heat exchanger assembly in the furnace housing in a manner such that in the operatively positioned furnace the secondary heat exchange portion of the heat exchanger assembly slopes downwardly toward a drain fitting installed at the combustion outlet end of the heat exchanger assembly. The drain fitting, in turn, is connected to a suitable condensate drain discharge conduit.
Fuel-fired forced air furnaces are typically manufactured in both vertical air flow configurations (both upflow and downflow) and horizontal air flow configurations to provide installation flexibility for a particular furnace line. Whether a vertical flow furnace or a horizontal flow furnace is needed is dictated primarily by the mechanical equipment space available for installation of the furnace. For example, if the available equipment space is a closet area, a vertical air flow furnace configuration is typically required. If, on the other hand, an attic space is available for concealing the furnace, a horizontal air flow furnace configuration is typically utilized.
In nonrecuperative furnaces (which, as mentioned above, do not generate condensate drainage in their heat exchanger structures during furnace operation) substantially identically configured heat exchangers of the same heating capacity may typically be installed by a given furnace manufacturer in a housing of one of its furnaces regardless of whether the furnace is designed to be operated in a vertical air flow orientation or a horizontal air flow orientation.
This has not been the case, however, in recuperative furnaces requiring condensate drainage from the secondary heat exchange portions of their overall recuperative heat exchanger assemblies. To provide proper condensate drainage for such recuperative heat exchangers it has heretofore been necessary to manufacture the heat exchanger assembly in one configuration for installation in vertical air flow furnaces, and in a different configuration for horizontal air flow furnaces. As will be readily appreciated, this requirement for producing heat exchanger assemblies in different shapes to accommodate both vertical and horizontal furnace installations undesirably tends to increase the overall manufacturing, inventorying and fabrication costs of a given furnace line.
It can be seen from the foregoing that it would be desirable to provide a recuperative heat exchanger assembly having a configuration permitting it to be operatively installed, in a manner permitting gravity drainage of combustion gas condensate therefrom, in either a vertical air flow furnace or a horizontal air flow furnace. It is accordingly an object of the present invention to provide such a recuperative heat exchanger assembly.